The present invention relates generally to the manufacture of optical fiber cable, and more specifically to the continuous manufacture of armored optical fiber cable in which the optical fiber is encased within an hermetically sealed small diameter metal tube.
One use of armored optical fiber cables is for communication transmission lines which are often many kilometers in length and exposed to a variety of mechanical and chemical environmental hazards. Because of the length of such cables and the difficulty and expense of field splicing optical fiber cables, it is desirable to manufacture such cables in long continuous lengths, rather than in relatively short discrete lengths. Additionally, it is desirable that the armor tube of armored optical fiber cables be strong and well sealed. Preferably, the armor should be made of a material, such as stainless steel, which is both strong as to its physical characteristics and resistant to chemical attack.
One problem associated with the manufacture of continuous lengths of armored optical fiber cable is that long lengths of optical fiber cannot easily be inserted longitudinally into the bore of similarly long ready-made seamless tube. Consequently, for the manufacture of long lengths of cable, the optical fiber must be introduced into the armor tube as the tube is formed, resulting in a longitudinal seam in the armor tube which preferably is sealed by some means.
A variety of approaches have been proposed for sealing the seam of the armor tube of an armored optical fiber cable. The predominate sealing method as shown by patents issued in the optical fiber cable art is the application of molten solder to the seam, whereupon the solder wicks into the joint and effects a seal. Soldering is disclosed as the preferred sealing means in such representative references as U.S. Pat. Nos. 4,508,423, issued to Winter et al.; 4,555,054 issued to Winter et al.; 4,557,559 issued to Winter; and 4,573,253 issued to Smith et al. Winter et al. '423 and '054 prefer to also use a heat-protective sheath surrounding the optical fibers at the soldering station to protect the optical fibers from the heat of soldering, with Winter et al. '054 disclosing this in the context of small diameter optical fiber cables. Winter '559 does not specifically describe the preferred soldering station. but refers to the soldering method of Winter et al. '423 as being suitable. Smith et al. discloses that it is preferred to pass the tube rapidly through molten solder, in contact therewith for only a fraction of a second, in order to reduce the risk of heat damage to the optical fibers. Hence, it would appear that there is an industry recognition of the risk of damage to the optical fibers from excessive heat during the seam sealing operation, and consequently the heretofore preferred method of effecting the seal was with solder, either in combination with a heat shield or with minimal exposure time to the solder.
Unfortunately, soldered seams do not possess as much strength and assurance of hermeticity as is desirable, because of the disimilarity of material between the tube and the solder, and because solder alloys in most cases are not as strong as the metal from which the tube is made, such as stainless steel.
It would be desirable to seal the seam by means of welding, such that the tube and the seam seal are of like material, resulting in a homogenous sealed tube of uniform high strength. Winter '559 mentions welding as an alternative sealing means, but provides no specific disclosure of welding means which would be suitable for sealing armored optical fiber cables. Likewise, U.S. Pat. No. 4,232,935 issued to Rhoner et al. generally discloses a welded seam made by a welding machine, but provides no speclfic disclosure of a suitable welding means in light of the recognized risk of heat damage to the optical fibers when sealing optical fiber cables. Hence, Winter '559 and Rhoner are believed to disclose only the desirability of welding the seam, but provide no teaching as to a welding apparatus which would actually accomplish the purpose of sealing an optical fiber cable without damaging the optical fibers therein.
The present invention does not suffer from the above-mentioned limitations of the conventional solder method of sealing small diameter optical fiber cable armor tubes, yet permits welding of the tube seam without damage to the optical fiber within, even in the absence of a heat shield material located between the optical fiber and the tube seam during welding. An important component of the present invention is a beam of laser light, which is used to weld the tube seam, which method of seam sealing is believed to be novel in the optical fiber cable manufacturing art. Use of a laser results in a relatively fine and smooth weld line on the outer surface of the optical fiber cable armor tube, and the inner surface of the tube remains smooth at the weld site. Furthermore. the heat applied by the laser is highly localized and intense, permitting welding of the seam without overheating the optical fiber located within the tube.